One fish, two fish, dead fish, new fish…

Hauling in the trawling net and getting white bins ready to receive the catch

As the sun goes down, the night crew readies themselves for an entirely different set of activities. On the docket for tonight are three 30-minute otter trawls (our net is 16 feet wide) at a depth of 600 meters. The replicate trawls at each depth and general location help to document the potential variability in any given area. Trawling at night brings whole new challenges to navigating the ship. Out here over the deepwater canyons, fishermen often set up floating longlines in hopes of catching pelagic species like swordfish, or bottom-dwelling fish like cod or monkfish. Though regional councils (established by the Magnuson-Stevens Fishery Conservation and Management Act of 1976 ) and federal agencies (NOAA National Marine Fisheries Service) regulate that these longlines must be equipped with radar reflectors on the endpoints, these can be difficult to detect. Regulations facilitate safe travel for everyone and aim to limit fishery harvests to sustainable levels.

The living organisms collected in the trawl net will serve different scientific purposes depending on the research group involved: each organism collected helps to verify and add detail to the photographs of underwater habitats taken by the ROV, all octocorals are sampled to investigate population and phylogenetics, all fish are preserved to have their stomach contents analyzed to investigate trophic dynamics, and everything (including fish) gives a tissue sample for isotope analysis as a second form of investigating trophic level relationships between organisms in these deepwater environments. Since so many research objectives overlap, all the scientists (and the ship’s crew) work as a team to ensure that everyone is able to collect high quality data to share.

All hands on deck! As soon as the trawl catch is brought into the lab, everyone jumps into action working together to sort and process the organisms as quickly as possible (before the next trawl comes up).

As soon as the catch is dumped into buckets and brought inside, everyone works together to roughly (fish, corals, crustaceans, etc.) and then finely (by species) sort the animals for processing.

When possible, Dr. Amanda Demopoulos collects 5 specimens of each species for her isotope analyses. Taking 5 specimens of the same species (rather than 1) in roughly the same size class, at the same depth and location helps to represent the variability of isotope values within populations.

This subsample of 5 long-nose shrimp is ready to have their muscle tissue isotoped.

By measuring the amount of both carbon and nitrogen stable isotopes an animal has in its tissues, she can estimate its food source (using Carbon) and to which trophic level it belongs (using Nitrogen). Animals higher up the foodchain (e.g. carnivores like larger fish or birds) have a higher ratio of N15 to N14 their tissues. Animals closer to the base of the foodchain (e.g. herbivores like some zooplankton) have lower ratios of N15 in their tissues.

(For more on Dr. Amanda’s research and background see Eric’s blog from Aug. 30th)

Different trophic levels are represented by a single species in this simple foodchain (Image credit: Dr. Amanda Demopoulos). The higher up an animal is found in the tropic levels, the higher the ratio of N15 to N14 isotope it will have in its tissues.